The high temperatures prevalent in modern steam turbines have led to numerous design problems. One of these problems, excessive thermal stress, has stimulated a number of design changes in order to minimize the effects of large temperature gradients across structural walls as well as to minimize differential expansion between various turbine components. Generally, high pressure and intermediate pressure fossil turbines are constructed with concentric inner and outer casings which are cylindrical in shape and which have as high a degree of radial symmetry as possible in order to reduce thermal stresses. The inner casing, which is free to expand radially, supports the high temperature blade ring in the high pressure blade path. The inner casing also serves as a pressure vessel thus permitting a relatively thin outer casing. After expansion across the turbine blading the lower temperature steam flows through the chamber formed between the inner and outer casings in order to reduce the temperature differential across the inner casing walls to moderate values. While these improvements have both reduced the magnitude of temperature gradients and the fluctuations in temperature gradients across structural walls, the double casing design presents additional problems of thermal stress.
Specifically, turbine inlet pipes, which carry high temperature steam through the outer casing and the inner casing to the blade rings of a turbine, are another source of thermal stress and can result in fracturing around the joints between the piping and the turbine casings. Under steady state conditions, a stable temperature gradient forms across the inner casing wall as one side of the wall if subjected to a relatively constant high temperature steam flow while the other side of the wall is subjected to a similar, but lower temperature, steam flow. This is illustrated by curve A in FIG. 1 wherein the temperature variation between the inside and outside walls of an inner casing is qualitatively described. However, when the incoming steam flow rate rises in response to an increased demand for power, the rate of heat transfer to the inside wall of the inner casing increases and the temperature gradient across the inner casing shifts as illustrated qualitatively by curve B in FIG. 1. This thermal shift results in differential expansion about the joints between the inlet piping and the turbine casings. Limited efforts have been made in the past to shield turbine casings as well as other stationary turbine elements from direct steam flows which are most responsible for inducing these thermal stresses. By way of example, with reference to FIG. 2, there is illustrated the provision of a shielding element 4 disposed about the inside surface of inner casing 6 of a turbine 9. This shielding is spaced a small distance from the casing wall in order to provide an annular gap which shields the casing from the high rates of heat transfer which would otherwise result from the rapid flow of steam through the turbine.
A different approach has been taken in the past in order to reduce thermal stress in the connection between the inlet pipe 12 and the outer casing wall 8. Pipe 12 terminates in a flexible bell connector 16 which is a tube-like member having three connections. A first weld connection joins pipe 12 to a first end 18 of connector 16. The connector 16 comprises an annular flange 20 which is secured to the outer casing 8 by a second weld connection. The second end 22 of connector 16 forms a tight but flexible connection with an inlet sleeve 28. Pressure seal rings 24 assure that high temperature steam does not escape the flexible connection joint and enter into chamber 32 as steam flows through the inlet sleeve 28 and into inner casing 6. As pipe 12 expands under conditions of increased steam flow the second end 22 of connector 16 may freely expand without stressing either the connection of flange 20 to the outer casing or the flexible seal ring connection between connector 16 and sleeve 28.
In the past, the provision of thermal shielding about casing walls and the use of flexible bell connectors as described above have provided some relief from the heat stress problems which occur in turbine casings. These design techniques however, have not been suitable for reducing thermal stresses at the connection 40 between the inlet sleeve 28 and the inner casing 6. For example, the extension of shielding 4 to fully line the inside of inlet sleeve 28 creates undesirable flow disturbances and is to be avoided. Even a limited extension of shielding 4 in the inlet sleeve as illustrated in FIG. 2 creates an undesirable flow disturbance at end point 38 of the shielding. Consequently, a solution has not heretofore been available for reducing heat stresses across the connection joining the inlet sleeve 28 and the inner casing 6.